Work and Pensions Secretary Iain Duncan Smith has signalled his opposition to an additional £10 billion of welfare cuts.
In a move that threatens to put him on a collision course with Chancellor George Osborne and other Cabinet ministers, Mr Duncan Smith warned that welfare was not "an easy target".
Mr Osborne raised the prospect of the further £10 billion of cuts in welfare spending - on top of the £18 billion reduction by 2014 - in last month's Budget.
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He said that was how much would have to be saved to avoid extra spending cuts for other departments.
But, asked whether he thought welfare cuts of that size were acceptable, Mr Duncan Smith said: "My view is it's not."
In an interview with The Times, the former Tory leader indicated that further savings could be made but stressed the need for a "balance of what we're trying to achieve".
"There is in my view no such thing as an easy target in welfare," he said.
"Some people think there is: until I show them where we spend the money.
"My view is that you have a responsibility to support people in difficulty. It's a prime concern of ours - we can't run away from that."